Cognitive impairment, mental retardation, and autism are devastating neurological components in a large number of patients with tuberous sclerosis (TS). Imaging studies and neuropsychological testing have traced these features to specific brain regions including the cerebral cortex and thalamus. In addition, morphological, histological, and electrophysiological analyses reveal that some neurons in TS exhibit abnormal morphology and function. Notably, TS is a developmental genetic disorder that manifests at birth and arises from mutations in Tsc1 and subsequent mTOR dysregulation. However, the role of Tsc1 at critical stages of brain development has not been determined. Neural circuits, established during brain development, underpin normal brain function and when perturbed causes insidious forms of neuropathology. However, whether anatomical correlates of cognitive dysfunction in TS include altered neural circuits has not been thoroughly investigated. My lab uses sophisticated mouse genetic approaches to combine conditional gene deletion using a modified allele of Tsc1 and cell marking with fine temporal control in vivo. Using these approaches, we will "mutate and mark" neurons in the thalamus during embryogenesis to ascertain how Tsc1 loss-of-function at distinct time points impacts the development of the thalamus. Preliminary data from our lab indicate that thalamic neurons are sensitive to Tsc1 deletion at mid embryogenesis, which results in increased pS6 levels, thalamic neuron doubling in size, aberrant thalamocortical projections, and seizures and repetitive grooming behaviors. We are augmenting these findings to identify the time periods where Tsc1 functions in brain development and to determine how rapidly brain disease occurs. Rapamycin has been proposed as a therapeutic agent to dampen mTOR, although how different populations of neurons respond to the timing and duration of rapamycin has not been determined. Our second aim tests the ability of the thalamus to respond to rapamycin treatment initiated at carefully chosen time points. Collectively, these experiments will test the hypothesis that Tsc1 has differential function in thalamic neurons that is tightly linked to developmental stage and process. These genetic tools will provide a valuable framework for understanding mechanisms involved in altered brain function associated with TS and will provide insight into developmental changes that may underpin TS. Importantly, because our genetic lines of mice can investigate morphology and circuitry relevant to specific brain regions, they can serve as framework to evaluate physiological and behavioral correlates of Tsc1 mutations in selective contexts and may be valuable substrates upon which to test therapeutic approaches relevant to ameliorating TS-related abnormalities in neurodevelopment and brain function.
Ultimately, proposed research is applicable to understanding the neurological component of TS, the cellular and molecular basis for TS, and may have direct clinical applications in terms of screening and testing therapeutic approaches in vivo. Patients who may benefit most from the genetics-based approach are children who exhibit early developmental involvement and patients with neurological deficits including cognitive impairment and associated autism-spectrum like disorders. The reason that these patients are most likely to benefit is that we are specifically and methodically testing the temporal role of Tsc1 in brain development while at the same time establishing an animal model system that can then be used to assay therapeutic compounds in a cell-type specific manner. The additional clinical benefit is that we will ascertain the stage of brain development in which Tsc1 loss of function impacts neural circuits and neuron morphology, which may provide insight into when patients should be screened for TS. An additional benefit of this research is that animal models will be established that will be instructive for screening compounds that impact specific populations of neurons. Finally, unraveling the brain regions, neuronal cell types, and neural circuits that cause cognitive abnormalities in many patients with TS will likely be a significant contribution to advancing TS research/knowledge.